398 RECENT PROGRESS IN PHYSICS. 



hydrosulphuret of ammonia, but Steinheil gilded them. The original 

 metallic mirrors can be used as moulds without any special prepara- 

 tion. Steinheil took great care to obtain the first deposit perfectly- 

 pure; for this purpose he made use of a separate decomposing cell, and 

 immersed a proof-plate first to ascertain whether the action was regular 

 and the precipitate even and rose-colored. Afterwards he placed the 

 mould in an ordinary, simple apparatus, and continued the operation 

 until the mirror was one inch thick. Projections, &c., he removed 

 every day with a file. As yet this process seems to have led to no 

 practical results. — (Dingler's Journal, vol. 99.) 



Electrotype copies of daguerreotypes, which had been previously 

 gilded, (Dingler's Journal, vol. 91, Philos. Magazine, September, 

 1843,) were proposed by Draper; but they appear to have led to no 

 result, or at least not to that desired. Still less practically successful 

 was Napier's process; he deposited copper upon linen cloth in order 

 to make it water-proof. The weight of this coating was intended to 

 to be only 12 ounces per square yard. — (Dingler's Journal, vol. 91.) 



In order to procure the solution of sulphate of copper required for 

 such purposes at a less price, Schubert advises (Dingler's Journal, vol. 

 81; Erdmann & Marchand's Journal fur Practische Chemie, 1841, No. 

 11,) to buy from coppersmiths the copper ashes (scale,) to mix them 

 with sulphuric acid of 15-20° Baume to the consistency of paste, and 

 let them remain thus for twenty-four hours upon plates of stone ware, 

 and then wash them with sulphuric acid of the same strength. This 

 last process is to be repeated until either the copper ashes are exhausted 

 or the acid is saturated. The former is the case when the ashes on 

 being moistened no longer become red, and the latter is seen from the 

 saturated color and the cessation of the acid reaction. Instead of fresh 

 sulphuric acid in the after washings, the fluid may be used which has 

 been partially exhausted in the electrotype process. 



§ 182. Gilding, silvering, etc., by means of galvanism. — It had already 

 been observed for some time that in certain cases metals precipitated 

 in a compact state and adhered firmly to pole wires immersed in 

 metallic solutions, and it had also been suggested that in this way an 

 entire coating of one metal upon another might be obtained; but De 

 la Eive first made a distinct application of this by showing how a solu- 

 tion of gold must be decomposed by means of galvanic currents, in 

 order to coat an object, when forming the negative pole, uniformly with 

 a thin layer of gold. 



De la Rive (Dingler's JournaljVol. 76; ComptesEendus,1840,p. 578,) 

 used a solution of chloride of gold — 40 to 80 grains metallic gold to 1 

 pound of water — in a bladder, and suspended it in water, acidulated 

 with "a few drops" of sulphuric acid, in which the zinc was placed; 

 the object to be gilded was connected with the zinc by means of a copper 

 wire, and immersed in the solution of gold; after a short time he took 

 it out, washed it, rubbed it well with a clean linen cloth and immersed 

 it again, and he repeated this several times. After a few immersions 

 he considered that the object was sufiiciently gilded, and that a longer 

 continuation of the operation was of no use. Among the reasons 

 which led him to this process he mentions the following: "3. The 

 property of the electric current of passing the more readily from a fluid 



